The present invention relates generally to battery charging systems, and, more particularly, to a system and method for fast charging a battery when a fast charging current source is above a minimum current level.
The use of a battery as a power source to power an electrical device is widespread. The battery provides the power necessary to operate the electrical device, and may be positioned proximate to, or housed within, the housing of the electrical device to be carried therewith. Because, when the electrical device is powered by a battery, no fixed connection between the electrical device and a permanent power supply is necessary to permit operation of the device, the electrical device need not be physically positioned proximate to a permanent supply of electrical power to permit operation of the device. Additionally, when the battery is carried along with, or within, the housing of the electrical device, the portability of the electrical device is enhanced.
However, a battery may only store a finite amount of energy to be converted into electrical power. Use of the electrical device discharges the battery, to thereby deplete the battery of remaining stored energy. As a result, after a certain period of use of the electrical device, the battery powering such device must be removed and replaced with a fresh battery.
Rechargeable batteries, e.g., nickel-cadmium type batteries, are widely available. A rechargeable battery, once depleted of stored energy, may be connected to a charging current source, and then recharged. Once recharged, the battery may be connected to power again an electrical device. Such a battery may be recharged up to five hundred, or even more, times, and is advantageous both for reasons of convenience and cost.
Various apparatus is available for recharging a rechargeable battery. Typically, such apparatus includes structure for supporting the rechargeable battery, and a means for electrically connecting the rechargeable battery to a charging current source to be charged therewith. The amount of time required to recharge the rechargeable battery is dependent upon the magnitude of the charging current supplied to the battery (and, additionally, the particular characteristics of the battery being recharged). Some types of battery charging apparatus supply a relatively small current to the battery, and an extended period of time is required in order to recharge the battery. Other types of battery charging apparatus supply a relatively large current to the battery (i.e., fast charge the battery) in order to permit the battery to be recharged in a short period of time. Still other types of battery charging apparatus permit either the relatively small current or the relatively large current to be supplied to the battery.
While, for reasons of convenience, supplying the battery with a relatively high charging current (i.e., fast charging) is advantageous, care must be exercised in order to ensure that the fast charging current source is disconnected from the battery once the batttery has been fully charged to its rated capacity. Overcharging of the battery with the fast charging current can result in damage to the battery, such as, for example, gassing, electrolytic venting of the battery, permanent loss of battery capacity, and physical damage to the battery.
Therefore, many of the known apparatus for charging a battery with the fast charging current include means for terminating fast charging of the battery once the battery is fully charged. For instance, various existing battery chargers include apparatus for implementing the following methods for determining when a battery has been fully charged: coulometric control, time control, pressure sensing, temperature sensing, incremental temperature cutoff, differential temperature cutoff, rate of temperature change, voltage magnitude sense, positive rate of voltage change, inflection point cutoff, voltage decrement cutoff, and negative rate of voltage change. These methods can provide signals responsive to a determination that the battery has been fully charged to terminate fast charging of the battery.
The above-mentioned methods of determining when the battery is fully charged responsive to measurements of battery voltage levels fail to account for battery characteristics of the particular battery undergoing charging. For example, battery characteristics vary according to battery age, battery manufacture, battery type, etc. Hence, a determination that a battery is fully charged responsive to measurements of battery voltage levels is frequently inaccurate.
Additionally, the above-mentioned methods of determining when the battery is fully charged responsive to measurements of battery temperature levels are operative on the premise that, once a battery is fully charged, continued charging of the battery with the fast charging current causes a rapid temperature increase of the battery. However, in some instances, the fast charging current supplied to the battery may be of a level too low to cause a rapid increase in battery temperature even after the battery is fully charged. Battery charging apparatus utilizing these methods, therefore, may not terminate fast charging of the battery, and the fast charging current will be supplied to the battery to continue charging thereof even after the battery is fully charged. As mentioned hereinabove, this continued charging can cause damage to the battery.
U.S. Pat. No. 4,649,333 to Moore, U.S. Pat. No. 4,692,5682 to Lane et al., and U.S. Pat. No. 4,712,055 to Houser, Jr., all disclose battery chargers which terminate fast charging of a battery when the battery temperature levels increase beyond a predetermined value.
Therefore, a battery charging apparatus which provides means for determining when the charging current supplied to the battery is of a level less than a charging current level which causes a rapid battery temperature increase upon full charging thereof is required.
Additionally, certain existing battery charging apparatus permits two or more batteries to be simultaneously charged. However, because the batteries are positioned in parallel to permit charging thereof, the current supplied to any individual battery is only a fraction of the total current supplied to the charging apparatus. The amount of current supplied to any particular battery is therefore dependent upon the number of batteries being charged simultaneously.
Typically, the battery charging apparatus is supplied current from a permanent power supply through appropriate coupling to a transformer interconnecting the power supply and the battery charging apparatus. Therefore, the amount of current supplied to a battery positioned in parallel with one or more other batteries is dependent not only upon the total number of batteries positioned to be charged simultaneously, but, additionally, upon the size of the transformer to which the charging apparatus is coupled to receive the current.
The current supplied to battery charging apparatus which may be adequate to fast charge a certain number of batteries may not be adequate to fast charge an increased number of batteries without causing the fast charge termination problems noted hereinabove. While the battery charging apparatus may be designed to permit fast charging of only a predefined number of batteries with current supplied through a transformer of a particular size, such charging apparatus would be of limited versatility.
What is needed, therefore, is battery charging apparatus which can determine the amount of current supplied thereto, and responsive to such determination, permit fast charging of only the number of batteries which may be suitably fast charged.